Description:

Objective:

Groundwater aquifers are the major, and sometimes
only, water supply in semiarid regions. Because rainfall in arid regions is
typified by
strong seasonality during which storms may be of very high intensity, fractures
and other macropore systems are known to play a major role in groundwater recharge.
In many arid regions, salt crust formation is common both on the ground surface
and inside macropores. Formation of these crusts is the result of extended
periods of time during which the evaporation potential far exceeds the annual
rainfall, and is attributed to evaporation, wetting and drying cycles, and
soil capillarity. Salts washed from the ground surface and from the walls of
macropores during recharge events are carried rapidly towards groundwater.
Because the salts are concentrated in these areas, a disproportionately high
volume of salts is added to the groundwater relative to the amount of rainfall.
Subsequent low rainfall periods then allow the evaporative potential of the
sun to draw salts back to these surfaces, again concentrating them. For these
reasons, use of a bulk soil salt concentration and precipitation records may
be very inadequate to predict the potential salt loading to groundwater in
arid regions. The objective of this research project is to create a mathematical
model to aid in analyzing the relative importance of competing processes.

Approach:

Salt accumulation rates on a fracture surface, and
subsequent transport to groundwater systems by episodic events is a complex
process. Although the
process and potential transport rates have been suggested by field observations,
a mathematical model to predict the rate of formation and transport potential
has not previously been formulated. The mathematical formulation of the problem
will require inclusion of at least four main driving forces: evaporation, osmotic
potential, capillarity, and hydraulic potential. Because these forces are not
linearly dependent, the interaction of the forces will be complex. A general
conservation of mass, momentum, and energy in a continuous media approach is
used to derive the general governing equations. Constitutive relations are
developed and adopted from the scientific literature. The resulting mathematical
model will be used to analyze the relative importance of those processes identified
in a series of laboratory experiments and fieldwork conducted in the Negev
Desert, Israel. Furthermore, the model will be used to design future experiments.

The perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.